Dave's E-size Plotter

This is Dave's plotter.

We spent 3 years designing our new house - on the computer. The builder, of course, would want the plans on paper, not in the computer. But rather than pay someone $100 or so to plot our house plans on paper, tightwad Dave decided he'd rather build his own plotter. So he and son Mike spent a year, perhaps a man-year of effort and many times more than $100 to design and build a plotter.

Although a plotter capable of producing D-size (24" x 36") plots would suffice for the house drawings, Dave decided on E-size (36" x 48") so it could be used on larger model drawings. Actually, because of the design, it can handle paper 36" wide by practically any length (useful for the wing of a really BIG model airplane!).

To draw lines, the pen is moved across the paper from side to side while the paper is moved forward and backward by a 36" long shaft ("capstan") running from side to side beneath the paper. Most plotters operate this way.

This image shows much of the mechanism. The pen is mounted on the carriage which rides on two round steel rails and is pulled from side to side by a wire running across the plotter, just behind the front rail. This wire runs over a pulley at each end, beneath the bed to a stepper motor (not shown) under the middle of the plotter.

Another stepper drives a toothed belt (orange-colored thingy) which turns a pulley (round gray thingy) connected to the capstan. Pinch rollers mounted to the square tube running across the front press the paper against the top of the capstan.

A relay on the carriage raises and lowers the pen to the paper. The relay gets its signal from electrical brushes contacting the two rails.

To minimize friction, the carriage rides the rails on small low-friction plastic rollers (dark gray). The carriage must be rigid but lightweight, thus the lightening holes in the aluminum sides. These holes, like most parts of the plotter, are homemade, not storebought.

The capstan drive is more clearly seen in this picture. The stepper motor drives a toothed pulley about 5/8" diameter. A toothed belt connects this pulley with a 1-1/4" pulley on the end of the capstan. Each time the stepper motor is stepped, it rotates about 1/800 of a revolution, so the capstan rotates 1/1600 rev. The capstan circumference is about two inches, so each step moves the paper 1/800 inch.

This view also shows better the pulley that the carriage wire runs over on its way to the carriage drive stepper motor buried in the middle of the plotter. The two gray wires near the pulley carry relay electrical signals to the carriage rails.

The wire driving the carriage is just behind the front round rail in this rear view.

This also shows the pinch rollers pressing the paper down against the capstan (black). They are mounted to spring brass plates and are raised and lowered by rotating the square steel tube running across the front of the plotter.

Astute observers may note there is only a single pen, which would seem to allow only a single line width and color. NOT SO! The software is written to stop and ask the operator to change pens when necessary. This, of course, requires someone to stand by and watch the drawing develop, but this is no hardship - it's really a whole bunch of fun to watch in action - watching the paper jerking fore and aft, the carriage running left and right and the pen clicking up and down as the lines are added to gradually form a completed drawing. At a pen speed of about 10 inches per second, it's not at all an unpleasant way to spend 15 or 30 minutes!

This, actually, is the second plotter built by Dave and Mike. The first was done as a home project while Mike was in high school and getting interested in electronics and computers. It was a small plotter (17 inches wide) with its own built-in microprocessor, RAM, EPROM, I/O, etc. to control its actions. Dave did the mechanical and electronic stuff (with Mike participating and learning), while Mike wrote the software for the microcomputer, in 8088 assembly language.

The E-size plotter grew from a need for the larger format and a desire to simplify the design. This one has much simpler electronics, relying on the home computer for its smarts. Again, Dave did the mechanics and electronics and Mike wrote the controlling software (this time in "C", running on the home PC).